1. Field of the Invention
The present invention relates to a displacement measuring device such as electrostatic capacitive, photoelectric and electromagnetic inductive encoders.
2. Description of the Related Art
In an electrostatic capacitive encoder mounted on a small length-measuring device such a linear scale, a scale is arranged opposing to a sensor head, interposing an air gap between them. For use in specifically sustaining the air gap between the scale and the sensor head, actually available systems include a system that accommodates the sensor head within a frame having three or more protrusions for sliding in order to oppose the sensor head to the scale, and a system that employs bearings.
These conventional systems are difficult to adjust the gap, however, when a more compact and smaller gap is required. Then, another system can be considered to slide the scale, directly contacting with the sensor head. This system requires a protection film for protecting electrodes on both the sensor head and scale. This protection film is required to have an excellent electrical insulation, planarity and wearability and a small coefficient of friction.
An insulating film, for example, a silicon oxide film, which is deposited through chemical vapor deposition (CVD), can be considered as such the protection film. A diamond-like carbon (DLC) film, which is deposited by plasma CVD, can achieve a much further excellent wearablity and much smaller coefficient of friction compared to the CVD silicon oxide film.
The following problems occur, however, when the DLC film is formed by plasma CVD as the protection film on the entire surface of the sensor head. First, there are steps of about 1 xcexcm depth/height on the surface of the sensor head and scale, due to electrode arrangement, worsening a tight adhesion of the DLC film to the surface. As a result, a sufficient durability can not be attained. In addition, the protection film is required to have a flat surface, but the surface of the DLC film reflects the steps at the electrode portion. Thus, a planarization process is also required for planarizing the DLC film. As a planarization technology, CMP (Chemical Mechanical Polishing) is well known, for example. However, since the DLC film has a high hardness, such a planarization process can not be applied easily to the DLC film.
The present invention has been made in consideration of the above situations and accordingly has an object to provide a displacement measuring device having an easily adjustable gap and such a sliding property that exhibits an excellent durability.
The present invention is provided with a displacement measuring device, which comprises a scale having a signal transfer section arranged along a measurement axis; a sensor head arranged opposing to and relatively movable to the scale, the sensor head having a signal transmitting section and a signal receiving section, for transmitting and receiving a signal via the signal transfer section; and a plurality of protrusions, located on a flat region in at least one of opposite surfaces of the scale and sensor head, for sliding on the other surface.
In the displacement measuring device according to the present invention, the protrusions are located on a plurality of positions in the flat region to avoid steps produced from arrangement of electrodes in the sensor head and scale. This allows the protrusions to tightly contact the ground material and slide with an excellent durability even though a DLC film is employed for the protrusions. In addition, it does not require any process for planarization as in the case of forming the DLC film over the entire surfaces of the sensor head and scale.
Preferably, in the displacement measuring device according to the present invention, the protrusions may particularly be composed of a laminated film consisting of a buffer film and a DLC film deposited thereon. Desirably, the buffer layer has a tight adhesion to the ground material and DLC film, and a larger hardness in some extent, though it is not required as high as that of the DLC film. Specifically, the buffer film may be formed of a film of insulator such as silicon oxide and silicon nitride formed by plasma CVD or spattering. The buffer film may also comprise a film of semiconductor such as polysilicon, if the protrusions are formed at locations where no electrode is arranged.
A period of time for film deposition is extremely long when a film thickness required to achieve a necessary gap is ensured only with the DLC film, though it can be reduced by use of a combination of the DLC film with another insulating film.
Further, when the protrusions are formed of the buffer and DLC films, the DLC film preferably extends from the protrusions over the electrode arrangement region. This allows the DLC film to serve as a protection film for the electrode arrangement region to prevent foreign materials from mixing into the electrode arrangement region.
Furthermore, when the electrode arrangement region in a sensor head substrate or scale substrate is previously formed to have a recessed region and electrodes are disposed within the recessed region, the protrusions are not required to have a much larger thickness. In this case, the protrusions can be formed of the DLC film alone.
Further, the protrusions formed of the buffer and DLC films are arranged on one of the sensor head and scale, the DLC film is preferably formed on the entire surface of the other as a protection film. This can achieve protection of electrodes in the other and attain an excellent wearable sliding property with a small coefficient of friction.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.